Correlating MOF-808 parameters with mixed-matrix membrane (MMM) CO2 permeation for a more rational MMM development

Consistent structure-performance relationships for the design of MOF (metal-organic framework)-based mixed-matrix membranes (MMMs) for gas separation are currently scarce in MMM literature. An important step in establishing such relationships could be to correlate intrinsic MOF parameters, such as CO₂ uptake and the CO₂ adsorption enthalpy (Q_st), with the separation performance indicators of the MMM (i.e. separation factor and permeability). Such a study presumes the availability of a platform MOF, which allows systematic comparison of the relevant MOF parameters. MOF-808 can take up the role of such platform MOF, owing to its unique cluster coordination and subsequent ease of introducing additional functional molecules. For this purpose, formic acid (FA) modulated MOF-808 (MOF-FA) was post-synthetically functionalized with five different ligands (histidine (His), benzoic acid (BA), glycolic acid (GA), lithium sulfate (Li₂SO₄) and trifluoroacetic acid (TFA)) to create a series of isostructural MOFs with varying affinity/diffusivity properties but as constant as possible remaining properties (e.g. particles size distribution). CO₂ uptake and CO₂ adsorption enthalpy of the MOFs were determined with CO₂ sorption experiments and Clausius-Clapeyron analysis. These MOF properties were subsequently linked to the CO₂/N₂ separation factor and CO₂ permeability of the corresponding MMM. Unlike what is often assumed in literature, MOF-808 CO₂ uptake proved to be a poor indicator for MMM performance. In contrast, a strong correlation was observed between Q_st at high CO₂ loadings on one hand and CO₂ permeability under varying feed conditions on the other hand. Furthermore, correlation coefficients of Q_st,15 and Q_st,30 (Q_st at 15 and 30 cm³ (STP)/g) with the separation factor were significantly better than those calculated for CO₂ uptake. The surprising lack of correlation between membrane performance and CO₂ uptake and the strong correlation with Q_st opens possibilities to rationally design MMMs and stresses the need for more fundamental research focused on finding consistent relationships between filler properties and the final membrane performance.